An echo canceller is a device that removes echo by subtracting the predicted value (echo prediction) from an echo-bearing signal that is composed of echo and a desired signal. The echo prediction is subtracted from the echo-bearing signal to remove the echo and this leaves mostly the desired signal. In practice the echo prediction is never perfect. Consequently there is usually some residual echo left over.
All echo cancellers require a reference signal. The reference signal is the known signal that is played out to a destination. For the application of acoustic echo cancellation, the reference signal is played out to a loudspeaker transducer to produce sound. At the same time the reference signal is directed to an adaptive filter to compute the echo prediction signal. The loudspeaker causes sound and much of this sound arrives at the microphone by way of both direct path echo and indirect echo reflections.
The arrangement of the acoustic echo canceller and the electrical and acoustic elements is illustrated in FIG. 1a. There are two inputs to an echo canceller, a reference signal and an echo-bearing signal. The echo canceller subtracts an estimate of the echo provided by 12 from the echo-bearing signal at 26 to create an echo-cancelled signal. The echo-cancelled signal is used as the means to adapt the coefficients of the echo model to improve the estimate of the echo. The signal that remains after cancellation consists of residual echo and the desired signal for transmission.
FIG. 1b shows an example of a typical echo canceller topology. Note this figure is identical to the digital signal processing part of FIG. 1a. In the figures that follow, the electrical and acoustic portion of the echo canceller arrangement is not shown. The embodiments of the invention pertain to the digital signal processing portion and the topology of that signal processing.
A problem with echo cancellation, especially for acoustic echo cancellation, is that the actual acoustic echo response can change suddenly. This change can result from many situations. For example, a person might move or put a hand in the echo path between the loudspeaker and the microphone. This causes a divergence between the echo canceller's prediction of the echo and the actual echo. The divergence may result in substantial residual echo that might be perceptible to a far-end listener.
In pragmatic acoustic echo cancellation situations, the echo coupling consists of direct and indirect coupling. The direct coupling is the echo that is due to the immediate acoustic echo path not including reflections off of walls and furniture. The direct coupling is equivalent to the echo that would be measured if the phone unit were in an anechoic chamber.
Disturbances to the echo path modify both the direct coupling and indirect coupling. In some instances, the prior art teaches that direct coupling should be fairly stable. There are many situations where the direct coupling response can be severely modified, for example, by obscuring the microphone hole, cupping a hand near a microphone to redirect sound into the microphone, and putting a big, hard, flat surface near the loudspeaker-to-microphone path. In these situations, the direct coupling portion of the echo is severely modified and the stable presumption for direct coupling is no longer true. For these situations, there is a high motivation to have a technique that detects severe echo path changes and makes fast adaptation to prevent echo from being heard.